Re: Memo on coding practices: strcmp() does not yield bool

I checked through the code and found no other problem cases.

> I've occasionally griped that I do not like the coding practice of
> writing
> if (strcmp(foo, bar))
> to mean
> if (strcmp(foo, bar) != 0)
> or its inverse
> if (!strcmp(foo, bar))
> to mean
> if (strcmp(foo, bar) == 0)
>
> My past objection to this has been purely stylistic: it's too easy
> to read these constructs backwards, eg to think "!strcmp()" means
> "not equal". However, I've now had my nose rubbed in the fact that
> this habit is actually dangerous.
>
> Up till just now, ruleutils.c contained code like this:
>
> bool tell_as = FALSE;
>
> /* Check if we must say AS ... */
> if (!IsA(tle->expr, Var))
> tell_as = strcmp(tle->resdom->resname, "?column?");
>
> /* more code... */
>
> if (tell_as)
> /* do something */
>
> This is subtly wrong, because it will work as intended on many
> platforms. But on some platforms, strcmp is capable of yielding
> values that are not 0 but whose low 8 bits are all 0. Stuff that
> into a char-sized "bool" variable, and all of a sudden it's zero,
> reversing the intended behavior of the test.
>
> Correct, portable coding of course is
>
> tell_as = strcmp(tle->resdom->resname, "?column?") != 0;
>
> This error would not have happened if the author of this code had
> been in the habit of regarding strcmp's result as something to compare
> against 0, rather than as equivalent to a boolean value. So, I assert
> that the above-mentioned coding practice is dangerous, because it can
> lead you to do things that aren't portable.
>
> I'm not planning to engage in a wholesale search-and-destroy mission
> for misuses of strcmp and friends just at the moment, but maybe someone
> should --- we may have comparable portability bugs elsewhere. In any
> case I suggest we avoid this coding practice in future.
>
> regards, tom lane
>

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